The present invention relates to a polarization beam splitter for guided light. It is used in integrated optics, e.g. in the field of optical fibre sensors or in the field of coherent information transmission by monamode optical fibres, in which the polarization state of the optical waves used is a very important perameter.
In these fields, polarization beam splitters are essential devices permitting the spatial splitting of transverse-electric modes (TE) and transverse-magnetic modes (TM) of the optical waves used.
Various guided light polarization splitters are already known, which are produced on electrooptical uniaxial materials and in particular LiNbO.sub.3 :Ti.
On the latter material, whose crystal axes are conventionally designated X, Y and Z, the X and Y axes both correspond to the ordinary refractive index of the material, whilst the Z axis corresponds to the extraordinary refractive index thereof, production has taken place of: directional couplers in propagation configurations along the X axis or Y axis (cf. document (1) which, like the other citations, is referred to at the end of the description), devices using bimodal interference methods (cf. document (2)), structures with Y-junctions (cf. document (3)), and splitters having exchanged ion waveguide parts (cf. document (4)).
Document (5) discloses a directional coupler with alternating delta beta electrode structure produced on a birefringent cut lithium niobate substrate and which can be used as a polarization splitter.
This directional coupler, diagrammatically shown in FIG. 1, comprises two light guides 2, 4 of constant width and two sections adjacent to one another having a length L/2, in which L represents the interaction length of the coupler. The light guide 2 (respectively 4) is provided with a control electrode 6 (respectively 8) in one of the two sections and a control electrode 10 (respectively 12) in the other section.
The control diagram of the coupler of FIG. 1 is partly shown in FIG. 2.
It is pointed out that a control diagram of a directional coupler is plotted in a reference frame, on whose abscissa axis is placed the parameter EQU delta beta.L/pi
in which pi is the well known number with a value of approximately 3.1416, L is the interaction length of the coupler and delta beta (designated with the usual Greek letters in the drawings) is the difference between the propagation constants respectively associated with the symmetrical and antisymmetrical propagation modes of the coupler.
On the ordinate axis of the reference frame, is placed the quantity L/lc, in which lc represents the coupling length of the coupler.
The control diagram of FIG. 2 constitutes a plurality of curves admitting the ordinate axis as the axis of symmetry. Curves I, III and V correspond to the crossed states of the coupler, designated by the symbol x on the axes of the reference frame. The curves II and IV (semicircles) correspond to the parallel states of the coupler, which are designated by the symbol=on the axes of the reference frame.
Examination of the matrix of electrooptical coefficients r.sub.ij of the lithium niobate shows that, for a given electrical field applied in the direction of the crystal axis Y and a transverse wave propagating parallel to the axis X, the delta beta phase displacements induced by the electrooptical effect on the fundamental transverse-electric (TE) and transverse-magnet (TM) modes are in a ratio close to 3.
Thus, on considering the coupler of FIG. 1 produced on an alternating delta beta structure on a lithium niobate substrate, whose surface is perpendicular to the crystal axis Y, the guided wave propagates parallel to the axis X, H being the representative point of the coupler in the absence of a field applied (cf. FIG. 2).
This point H is located on the ordinate axis of the control diagram (delta beta=0). If an electrical field parallel to the axis Y is applied, the coupler admits two representative points A and B, respectively relating to the guided modes TE and TM.
From the statement made hereinbefore is taken: EQU HB=3HA
If the point H and the value of the field applied are chosen such that the point A forms part of the curve I and the point B forms part of the curve II, respectively linked with the crossed and parallel states of the coupler, a splitting will be obtained of the polarized waves TE and TM. As the optical wave is injected at the output 14 of the guide 2 (FIG. 1), the polarized wave TM will be obtained at the end 16 of said guide 2 and the polarized wave TE will be obtained at the end 18 of the guide 4 (located at the side of the end 16).
This leads to two important disadvantages: correct operation requires a very precise choice of the point H and therefore the ratio L/lc of the coupler, the constructional tolerances being very narrow; the reasoning given hereinbefore implicitly presupposes that the curves I, II, . . . , V of the control diagram are the same for the TE and TM polarizations, which is not generally the case, the coupling lengths lc being different for the guided waves TE and TM.
Document (6) discloses a polarization splitter diagrammatically shown in FIG. 3. This splitter comprises a phase shifter 20, which interconnects two 3dB directional couplers 22, 24. By appropriately polarizing this known splitter, it is possible to respectively obtain at the two outputs 26 and 28 of the coupler 24, the TE and TM modes of an input lightwave arriving at one 30 of the two inputs of the coupler 22.
However, this known splitter has the disadvantage of lacking compactness, its length being approximately 2 cm or more.